Is Gravity RANDOM Not Quantum?

Summary

This episode explores the possibility of a classical theory of gravity compatible with quantum mechanics, challenging the quest for quantum gravity. Physicist Jonathan Oppenheim proposes 'post-quantum gravity,' suggesting gravity may be inherently random, not quantum. The theory addresses inconsistencies between general relativity and quantum mechanics, offering a new perspective on unifying these fundamental physics theories and their implications for quantum information and determinism.

Takeaways

• 🌌 The script discusses the ongoing quest for a unified theory in physics, specifically quantum gravity, questioning if gravity is inherently quantum or just appears messy or random.
• 🧠 It introduces the post-quantum gravity hypothesis by Jonathan Oppenheim, suggesting gravity might not be quantum but could be made compatible with quantum mechanics through the addition of randomness.
• 🔬 The script contrasts general relativity, which describes large-scale phenomena like gravity, with quantum mechanics, governing the small-scale behavior of matter and energy.
• 🔄 The conflict between these two theories is highlighted, as they are both verified to high precision but seem fundamentally incompatible, leading to the search for a theory of quantum gravity.
• 🤔 The possibility of a classical theory of gravity that works alongside quantum mechanics is explored, challenging the assumption that gravity must be quantized.
• 📚 Einstein's field equations of general relativity are explained, which equate the geometry of spacetime with the matter and energy within it.
• 🌐 The script delves into the complexities of coupling classical gravity with quantum theory, discussing the issues with semiclassical gravity and the expectation value of the stress-energy tensor.
• 🚀 The concept of a singular spacetime shaped by quantum superpositions of mass and energy is critiqued for its inconsistencies with quantum mechanics.
• 💥 The idea of adding noise to gravity itself to create a singular classical spacetime that allows quantum objects to behave quantumly is presented as a potential solution.
• 🔮 Oppenheim's theory of post-quantum gravity is described, which includes a fluctuating gravitational field that could resolve issues with the uncertainty principle and the measurement problem.
• 🎲 The script touches on the radical implications of post-quantum gravity, including the abandonment of determinism and the potential resolution of the black hole information paradox by allowing for the destruction of quantum information.

Q & A

What is the holy grail of theoretical physics?

The holy grail of theoretical physics is to find the long-sought theory of quantum gravity, which would unify the principles of quantum mechanics and general relativity.

What are the two great theories in physics that explain almost everything?

The two great theories are general relativity, which describes space, time, and gravity on the largest scales of the universe, and quantum mechanics, which describes atoms, matter, and the smallest scales.

Why do general relativity and quantum mechanics appear to contradict each other?

General relativity and quantum mechanics appear to contradict each other at the most fundamental level because they are each verified to astonishing precision but describe the universe in very different ways, with general relativity being a classical theory and quantum mechanics being inherently probabilistic.

What is the assumption hidden in the term 'quantum gravity'?

The term 'quantum gravity' assumes that the solution to unifying quantum mechanics and general relativity is to 'quantize' gravity, making it work correctly alongside quantum mechanics.

What is the post-quantum gravity hypothesis proposed by Jonathan Oppenheim?

The post-quantum gravity hypothesis suggests that gravity is not quantum but rather has an element of randomness or 'noise' that allows it to be compatible with quantum mechanics without needing to quantize spacetime itself.

What are the Einstein field equations?

The Einstein field equations are a set of 10 partial differential equations that equate the geometry of spacetime, described by the Einstein tensor, with the distribution of matter and energy, described by the stress-energy tensor.

How does the concept of semiclassical gravity relate to the discussion of quantum and classical gravity?

Semiclassical gravity is an approach where spacetime curvature is defined by the expectation value of the stress-energy tensor, allowing for quantum matter to influence a classical spacetime. It has been historically successful but faces issues when considering quantum superpositions.

What is the issue with using semiclassical gravity to describe a quantum object in superposition?

The issue with semiclassical gravity is that it leads to a singular spacetime geometry defined by the expectation value of the quantum object's location, which can result in odd behaviors, such as objects appearing to be attracted to nothing, and violates the uncertainty principle.

How does the addition of noise to gravity in post-quantum gravity resolve the issues with semiclassical gravity?

In post-quantum gravity, the addition of noise to the gravitational field allows for a probabilistic interaction between quantum objects and spacetime, preventing the violation of the uncertainty principle and allowing quantum objects to maintain their quantum behavior.

What is the radical aspect of Oppenheim's post-quantum gravity theory?

The radical aspect of Oppenheim's theory is that it introduces true randomness into the gravitational field, which has implications for the determinism of physics and the conservation of quantum information.

What is the potential consequence of allowing quantum information to be destroyed in post-quantum gravity?

Allowing quantum information to be destroyed by random fluctuations in the gravitational field could resolve paradoxes such as the black hole information paradox and provide a consistent way for classical spacetime to evolve with quantum matter.

Outlines

00:00

🔬 The Quest for Quantum Gravity and Postquantum Gravity Hypothesis

The script introduces the ongoing search for a theory of quantum gravity, which aims to reconcile general relativity and quantum mechanics. It presents the idea that gravity might not be quantum at all, but rather exhibit a kind of randomness as suggested by Jonathan Oppenheim's postquantum gravity hypothesis. The paragraph discusses the conflict between general relativity, which describes the large-scale universe, and quantum mechanics, which governs the small scale. It also touches on the challenges of unifying these theories and the potential need to rethink our approach to gravity's quantum nature.

05:03

🌌 Einstein's General Relativity and Quantum Mechanics Dilemma

This paragraph delves into the specifics of Einstein's field equations from general relativity and contrasts them with the probabilistic nature of the Schrödinger equation in quantum mechanics. It explains the classical nature of the Einstein equation and the quantum behavior of particles and fields, highlighting the difficulty in merging these two frameworks. The paragraph also explores the concept that classical behavior might emerge from a multitude of quantum interactions, suggesting that the classical stress-energy tensor could be derived from quantum entities.

10:10

📚 The Challenge of Unifying Quantum Mechanics with Classical Gravity

The script discusses the challenges of creating a quantum version of the Einstein tensor and the idea proposed by Jonathan Oppenheim that spacetime might remain classical while matter and energy are quantum. It uses the example of the Earth's atoms and their quantum uncertainty to illustrate how a classical stress-energy tensor could emerge from quantum components. The paragraph also examines the implications of treating the gravitational field as a superposition of possible spacetimes and the problems that arise when trying to reconcile this with Heisenberg's uncertainty principle.

15:12

🌐 Post-Quantum Gravity: A Solution with Intrinsic Randomness

The final paragraph introduces Oppenheim's post-quantum gravity theory, which incorporates a type of randomness into gravity itself to allow for a singular classical spacetime that is compatible with quantum mechanics. It explains how this randomness prevents the violation of the uncertainty principle and allows for the destruction of quantum information, potentially resolving paradoxes like the black hole information paradox. The script concludes by reflecting on the implications of this theory for the unification of physics' fundamental theories and ends with a promotional note about a limited edition Desktop & Gaming Mat.

Keywords

💡Quantum Gravity

Quantum gravity is the theoretical framework that seeks to unify general relativity, which describes the large-scale structure of the universe, with quantum mechanics, which governs the behavior of particles on the smallest scales. In the video, the concept is central to the discussion, as it explores whether gravity itself needs to be quantized to align with quantum mechanics or if a new approach, like post-quantum gravity, is needed.

💡General Relativity

General relativity is Einstein's theory describing gravity as the curvature of spacetime caused by mass and energy. It's one of the two major pillars of modern physics, explaining phenomena on cosmic scales. The video explains how general relativity is a classical theory, which contrasts with quantum mechanics, and the ongoing challenge is how to reconcile these two frameworks.

💡Quantum Mechanics

Quantum mechanics is the branch of physics that deals with the behavior of particles at the atomic and subatomic levels. It introduces concepts like superposition and quantization, which seem incompatible with the smooth, continuous nature of general relativity. The video's discussion centers on the tension between quantum mechanics and classical theories like general relativity.

💡Superposition

Superposition is a principle of quantum mechanics where a particle can exist in multiple states or locations simultaneously until it is observed or measured. The video uses the example of a 'quantum Earth' in a superposition of locations to illustrate the challenges of combining quantum mechanics with classical gravity.

💡Stress-Energy Tensor

The stress-energy tensor is a mathematical object used in general relativity to describe the density and flux of energy and momentum in spacetime. It is central to Einstein's field equations. The video discusses how the classical stress-energy tensor, which represents matter and energy, might emerge from quantum components.

💡Einstein Equation

The Einstein equation, part of general relativity, relates the geometry of spacetime (described by the Einstein tensor) to the distribution of matter and energy (described by the stress-energy tensor). The video highlights the challenge of making this equation compatible with quantum mechanics and whether only one side of the equation needs to be quantum.

💡Semiclassical Gravity

Semiclassical gravity is an approach that attempts to incorporate quantum mechanics into general relativity by treating spacetime as classical but influenced by quantum matter. The video explains how this approximation works and its limitations, particularly when dealing with quantum superpositions.

💡Heisenberg's Uncertainty Principle

Heisenberg's uncertainty principle is a fundamental concept in quantum mechanics stating that certain pairs of physical properties, like position and momentum, cannot be simultaneously known with arbitrary precision. The video discusses how classical gravity might violate this principle, posing a challenge to unifying quantum mechanics and general relativity.

💡Decoherence

Decoherence is the process by which a quantum system loses its quantum behavior and becomes effectively classical due to interactions with its environment. The video describes how the random fluctuations in post-quantum gravity could cause decoherence, collapsing the superposition of quantum states like that of the 'quantum Earth.'

💡Post-Quantum Gravity

Post-quantum gravity is a theoretical concept proposed by Jonathan Oppenheim, suggesting that gravity might not be quantum but rather classical with added randomness or noise. The video explores this idea as an alternative to traditional quantum gravity theories, proposing that randomness in gravity could reconcile it with quantum mechanics.

Highlights

Introduction of a new product at the merch store with more info at the end of the episode.

The theoretical pursuit of quantum gravity, questioning if gravity is quantum or just appears messy or random as per Jonathan Oppenheim's postquantum gravity hypothesis.

General relativity and quantum mechanics, the two great verified theories in physics that seem to contradict each other at a fundamental level.

The conflict between quantum mechanics and general relativity and the efforts to unify them into a single theory.

The assumption that gravity must be quantized to work with quantum mechanics and the possibility that this assumption might be incorrect.

Einstein's field equations and the concept that spacetime geometry is equated with its contents.

The distinction between the classical nature of the Einstein equation and the quantum nature of the Schrödinger equation.

The idea that classical behavior arises from a statistical limit of large numbers of quantum interactions.

The challenge of creating a quantum analog for the classical stress-energy tensor in the Einstein equation.

Jonathan Oppenheim's proposal that only the right side of the Einstein equation is quantum, not the left, suggesting a fundamentally classical spacetime.

The thought experiment involving measuring the position of the Earth's center of mass and its implications for the classical stress-energy tensor.

The problem with semiclassical gravity and its inconsistency with quantum mechanics when dealing with quantum superpositions.

The proposal of a classical spacetime in superposition with its contents as an alternative to semiclassical gravity.

The violation of Heisenberg's uncertainty principle in the context of a classical gravitational field and the quantum double-slit experiment.

The concept of adding noise to gravity itself in post-quantum gravity to allow for a singular classical spacetime that is compatible with quantum objects.

The feedback mechanism in Oppenheim's theory where the noisiness of the gravitational field affects the quantum matter generating the field, leading to decoherence.

The resolution of the uncertainty principle issue in the double-slit experiment through the introduction of a noisy gravitational field.

The radical aspect of post-quantum gravity that abandons determinism and allows for the destruction of quantum information.

The potential resolution of the black hole information paradox through the allowance of quantum information destruction in Oppenheim's theory.

The speculative nature of post-quantum gravity and its significance as a new direction in the pursuit to unify quantum mechanics and general relativity.

Announcement of a limited edition Desktop & Gaming Mat with a wormhole design and its features.

Transcripts

Hey everyone, we have brand new product at the  merch store. More info at the end of the episode.

The holy grail of theoretical physics  is to find the long-sought theory of  

quantum gravity. But what if this  theory is as mythical as the grail  

of legend? What if gravity isn’t weirdly  quantum at all, but rather … just a bit  

messy? Or random? So says the postquantum  gravity hypothesis of Jonathan Oppenheim.

We have two great theories in physics that  together appear to explain almost everything.  

There’s general relativity–the theory of space  and time and gravity and the largest scales of  

the universe. And there's quantum mechanics–the  theory of atoms, matter, and the smallest scales.  

These theories are each verified to astonishing  precision, and yet seem to contradict each other  

at the most fundamental level. We’ve talked about  these conflicts in the past, and the efforts to  

resolve them by unifying quantum mechanics  and general relativity into a single theory.

We usually think of this master theory as “quantum  gravity”, but that name hides an assumption:  

the assumption that the solution is to “quantize  gravity” in order to work correctly alongside  

quantum mechanics. But after nearly 100 years  of very smart people thinking about this,  

we still don’t know how to make gravity quantum.  Well, what if we’ve been going about it wrong  

all along? What if gravity simply isn’t quantum?  Today we will ask whether it’s possible to come  

up with classical theory of gravity which is  compatible with all the weirdness of quantum  

mechanics? According to physicist Jonathan  Oppenheim, the answer is basically yes,  

but some additional weirdness must be added to  gravity and its interactions: it must be random.

Before we get into the type of “randomness”  gravity needs to make such a theory work,  

let’s discuss what happens when one naively  tries to couple classical gravity as we  

know it to quantum theory. Our starting point is  Einstein’s famous equations of general relativity.

Here we have the Einstein field equations. We have the Einstein tensor, which describes the  

geometry of spacetime, equalling a bunch of  constants times the stress-energy tensor,  

which describes the matter and energy  inside that spacetime. By the way,  

the Einstein field equations are pluralized  because these tensors are multi-part objects–it’s  

really a set of 10 partial differential  equations. But despite the name it’s ok  

to think of it as a single equation–it equates  the geometry of spacetime with its contents. As  

John Archibald Wheeler put it, space tells  matter how to move, matter tells space how  

to curve. In other words, the right side  defines the dynamics–how matter will move,  

while the left is the mass-energy that gets moved.  So let's just call it the Einstein equation.

The Einstein equation is a classical  equation. Despite being complicated,  

its parts are regular numbers and vectors–for  example, the stress-energy tensor has things  

like momenta, energy, and mass of objects  in the universe, while the Einstein tensor  

describes a smoothly varying field with a  well-define and singular value everywhere.

The Einstein equation of general relativity  describes spacetime and gravity, while the rest  

of physics is described by quantum mechanics and  the Schrodinger equation. The Schrodinger equation  

is distinctly not classical–it does not describe  precise properties of objects but rather tracks  

the evolution of the wavefunction–the  fuzzy distribution of probabilities,  

representing what might be observed when a  measurement is made. Its subjects–systems of  

quantum particles and quantum fields–are  quantized, in that they tend to jump  

between discrete states rather than move or vary  smoothly like a classical object or field would.

We tend to think that our classical, macroscopic  world arises as the combination of countless  

quantum interactions–we’d say that classical  behavior is a statistical limit of large numbers  

of quantum interactions. In that spirit, many  physicists believe the general relativity and  

the Einstein equation are not fundamental,  but rather emerge from quantum foundations.

And that's understandable, because we do know that  matter and energy are fundamentally quantum. There  

must be quantum fields and quantum particles that  somehow work together to produce the classical  

stress-energy tensor. Those quantum entities  do all the weird quantum stuff, like existing  

in superpositions of being in multiple states  at once and having discrete energy levels.

So the classical stress-energy tensor on the  right of the Einstein equation should emerge  

from some quantum analog. Standard approaches to  unifying quantum mechanics and general relativity  

have been to try to make the left side of  the equation quantum also–effectively to  

find a quantum version of the Einstein tensor  representing a quantum spacetime geometry. But  

our long difficulties in making that work is  what led Jonathan Oppenheim to ask–what if  

only the right side of the Einstein equation  is quantum but not the left? What if matter  

and energy can be quantum, but the spacetime  they live in really is fundamentally classical?

The challenge here is that both sides of  the Einstein equation have to be the same  

type of mathematical object. So if the spacetime  represented by the Einstein tensor is classical,  

then we should probably understand how a  classical distribution of mass and energy  

defined by a classical stress-energy  tensor can arise from quantum parts.

Take the Earth for example. Each of its  countless atoms has a bit of quantum  

uncertainty in its position. Each atom is  in a superposition of multiple places at  

once until measured. If you were to  measure one atom, its location would  

resolve randomly within a small range  defined by the uncertainty principle.  

But each different possible location for that one  atom gives a different stress-energy tensor for  

that atom, and so a different spacetime geometry  due to the gravitational field of that atom.

So what if you try to measure the position of  the entire Earth by finding its center of mass?  

Think of that as measuring the position  of every single atom in unison. If those  

positions all get defined randomly, then any  significant variations cancel out. It’s like  

flipping a coin a kajillion times–you tend  towards roughly end up with equal numbers  

of heads versus tails. In the same way, each  time you measure the center of mass of the  

Earth you’ll get the same position–accounting  for its own movement of course. You find the  

same stress-energy tensor and so the  same gravitational field each time you  

look. This is why the stress-energy tensor for  macroscopic objects can be treated as classical.

What about describing a classical gravitational  field for a truly quantum object? Let's look at  

two options: we can either write down all the  possible stress-energy tensors for all possible  

locations of that object. The Einstein equation  could then be used to find a whole family of  

possible gravitational fields for each of  those. A superposition of many mass-energy  

distributions leading to a superposition of  possible spacetimes. While the superposition  

part of this is a quantum phenomenon,  each spacetime in the superposition is  

otherwise classical–for example, its smoothly  varying, without discrete, quantized states.

Alternatively, we could say that  there’s just one spacetime which is  

somehow uniquely defined by the superposition  of all of those mass-energy distributions. As  

though the effect of matter on the  fabric of spacetime is determined  

by every possible configuration  that quantum matter might be in.

Both of these approaches are problematic. Let’s  start with the second one I described–a singular  

spacetime shaped by the quantum superposition  of mass and energy that it contains. In fact,  

this is historically the most successful approach  to introducing quantum mechanics into general  

relativity. It’s called semiclassical gravity,  and it’s how Stephen Hawking figured out that  

radiation leaks out of black holes. The idea is  that spacetime curvature is defined by something  

called the expectation value of the stress-energy  tensor. The expectation value is just the average  

measurement value you would get if you were  to measure a quantum system multiple times.  

It better reflects the global wavefunction than  does a single, random measured realization of it.

In our example of the Earth, we saw that  the location of the center of mass doesn’t  

depend on the quantum uncertainty of  its component atoms. That’s basically  

the same as saying its classical  stress-energy tensor is equal to  

the expectation value of the total quantum  stress-energy tensor of all of its atoms.

But consider a truly quantum object–say, a quantum  version of the Earth that really could exist in  

a superposition in which there are two locations  for its center of mass. If you were to measure the  

location of the quantum Earth it would resolve  to, say, left or right. The expectation value  

of its location prior to measurement would be  halfway in between. In semiclassical gravity,  

that in-between location defines the singular  spacetime geometry and the gravitational field.

In regular classical gravity, objects like apples  fall towards the center of mass. In semiclassical  

gravity apples fall towards the expectation  value of the center of mass. In the case of the  

quantum Earth in superposition it falls halfway  between the two possible locations. In fact,  

each version of Earth should also fall  towards that inbetween point. For those  

living in one of those superpositions  unable to see the other superposition,  

the Earth would appear to be attracted towards  nothing. Needless to say, this seems odd.

Semiclassical gravity was always  meant to be an approximation,  

valid only in specific circumstances.  But this thought experiment rules it  

out as a consistent unification of quantum  mechanics and classical general relativity.

OK, moving right along. The other option I  mentioned for gravity being classical-ish  

is for there to be a different classical  spacetime corresponding to each possible  

mass-energy distribution in our  quantum stress-energy tensor.

If our quantum Earth is in a superposition of  two locations, then instead of having a single,  

classical spacetime geometry representing the  average of the two, we might have a superposition  

of two spacetime geometries–one for each of the  two mass-energy distributions. In this case,  

our falling apple wouldn’t fall towards  the average center of mass. Instead,  

it would fall to one or the other  possible locations randomly.

This sounds more promising because now  we have apparently random motion under  

gravity that tracks the quantum  randomness in the position of the  

source of that gravity. But there’s  actually a big problem here also.

This time we violate a sacred  tenet of quantum theory:  

Heisenberg's uncertainty principle. The  argument is due to Feynman, Aharonov,  

and Rohrlich, and in somewhat  simplified form goes like this.

We run our good-ol’ fashioned double slit experiment. A  beam of quantum particles is shot at a pair of  

slits. The quantum properties of the matter allow  each particle to be in a superposition of paths,  

passing through “both” slits at the same time.  The wavefunctions of the particles interfere  

with themselves and produce an interference  pattern on the screen placed after the slits.  

The shape of the interference pattern tells us  the wavelength of the particle’s wavefunction,  

which is equivalent to knowing it’s momentum.  But according to Heisenberg's uncertainty  

principle this means one shouldn’t be able  to determine the particle position. So if  

we measure the interference pattern we can’t  know which slit the particle went through.

But what if we try to cheat by placing a test  mass betweenf the slits. When our quantum  

particle passes through one or the other of the  slits then it will exert a stronger gravitational  

pull on the test mass compared towards  that slit. So the response of the test  

mass should identify which slit was traversed  without disturbing the interference pattern,  

allowing us to simultaneously  measure both position and momentum.

This method of cheating Heisenberg only works  if the gravitational field of the particle is  

localized and classical–i.e. the field really  does pass through one slit or the other.  

This has been taken as a solid argument  for why gravity can’t be truly classical,  

even if it’s allowed to be in superposition.

So far so bad. We found that a singular,  classical spacetime geometry that depends  

on the collective quantum state of  its contents doesn’t behave sensibly  

for quantum superpositions of matter. But an  otherwise-classical spacetime geometry that  

is in superpostion along with the  superposition of its contents will  

betray information about its contents in a  way that violates the uncertainty principle.

But there is a way to have a singular,  classical spacetime that allows its quantum  

contents to still behave like quantum  objects. The trick is to add … noise.  

To add a type of randomness to gravity  itself. In Jonathan Oppenheim’s theory,  

which he calls post-quantum gravity–we still  have a singular spacetime–one gravitational  

field–but now it fluctuates randomly  at every point. And the distribution of  

values of those fluctuations reflects the quantum  superposition of the matter generating the field.

Another way to think of it is like  this. In regular general relativity,  

when two objects interact gravitationally, they  sort of learn each other’s location based on the  

direction of the gravitational field. It’s that  perfect learning of position that can violate  

the uncertainty principle. But in post-quantum  gravity, gravitationally interacting objects  

don’t “learn” precise positions, but rather  a probabilistic distribution of the possible  

positions that depend on the position  wavefunction of the sources of gravity.

Let’s see how this works with the example  of quantum Earth in superposition. The apple  

starts falling generally towards the Earth, but  due to the fluctuations in the gravitational  

field you can’t at first tell whether it’s  falling more left or more right. In fact,  

those fluctuations cause the falling  apple to take a sort of random walk,  

sometimes drawn more to the left  superposition, sometimes more to the right.

Let's see what the Earth is doing as the  apple falls? Well, in Oppenheim’s theory,  

there’s feedback between the noisiness of the  gravitational field and the quantum matter that  

generates that field. So the quantum matter of  the Earth gets its own random fluctuations. These  

fluctuations slowly destroy the superposition–they  cause decoherence–basically, they collapse the  

wavefunction of the quantum Earth. This causes  the Earth to end up at a single location. In a  

sense, with every interaction between the  apple and the Earth’s gravitational field,  

the apple conducts a very gentle  measurement of the Earth’s position.

The strength of these measurements increases as  the apple gets closer to the Earth. Over the fall,  

Earth and apple become correlated, and the Earth’s  position becomes defined. If the apple fell more  

to the left, that’s where the Earth’s center of  mass will end up, if the the right, the right.

This fundamental randomness also resolves the  issue with the uncertainty principle when we  

tried to use gravity to measure the path in the  double-slit experiment. If the gravitational field  

of the particles in the beam are noisy, and that  noisiness reflects the superposition of traveling  

through both paths, then we can no longer use that  field to perfectly identify which path was taken.

Perhaps the most radical aspect of  Oppenheim’s idea is that it throws  

away determinism. It requires that this noise  in the gravitational field is truly random.  

And perhaps the most radical consequence of  that is that it allows for the destruction  

of quantum information. Conservation of  quantum information is thought by many to  

be among the most fundamental underpinnings  of quantum mechanics–that without it quantum  

mechanics is inconsistent. At the same time,  insisting on the preservation of information  

led to various challenges, like the black  hole information paradox. If you allow  

quantum information to be destroyed by random  fluctuations, then no such paradox exists.

So, it seems Oppenheim’s theory can describe  in a consistent way how classical spacetime  

evolves with superpositions of quantum matter,  and also how quantum matter itself evolves under  

the interaction with a classical spacetime.  Post-quantum gravity isn’t likely to be our  

final theory, and it may or may not be on the  right track. But it’s very interesting to see  

that there are still new directions to solving  this century-old problem. It give us hope that  

we’ll eventually unify our two great theories.  Will it be quantum or post-quantum gravity?  

As long as we figure it out, I’m cool living  in either a quantized or a random spacetime.

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